| In this dissertation, the fabrication and conditions optimization, microstructure and superconducting properties of Bi2212 film thins by Pechini sol-gel method were researched. High quality epitaxial Bi2212 superconducting thin films were fabricated by Pechini sol-gel method by controlling sintering temperature and time using non-toxic and economic inorganic salts as reactants to study their mictrostructure and superconducting propertied. By the optimization of preparation conditions, the effect of heating rate with different stages on phase composition, microstructure and superconductivity of precursor film and Bi2212 thin film has been comprehensively investigated. Bi2212 films were deposited on different substrates, and the effect of different substrates on growth condications and stress-strain induced by mismatch betwwen thin films and substrates on superconducting properties of the thin films were investigated. The main contents of thesis work are as follows:Bi2212 superconducting thin films were prepared on SrTiO3(100) (STO) single-crystal substrate by Pechini sol-gel method. The key to successful design experiment was to substitute chemical bonds formed in the esterification of EDTA complexes and EG for instable hydrogen bonds, and the amorphous thin film sintered at 400℃ was obtained due to the improvement of stability of Pechini sol, avoiding the segregation of composition. Sintering temperature and time of Bi2212 thin films due to the stable and uniform distribution of composition were significantly reduced and shrank, respectively. Bi2212 phase formation region was obtained from 821℃/55min to 827℃/13min and the highest Tc.onset value was 95K.Through the optimization of preparation conditions, it discovered that the optimal pre-heating rate was 2.0℃min-1. Below this pre-heating rate, partial composition segregation occurred in the precursor films that resulted in dispersion of impurity phases in sintered thin films and caused the degradation of superconducting properties. While above this pre-heating rate, the surface morphology of thin films was destroyed, as the macroscopic defects, such as cracks and holes caused by excessive gas emissions organic matter decomposition process and the sharp increasing in roughness of precursor film were observed. Through the optimization of the heating rate at sintering stage, the range of 3.0-3.25 ℃min-1 was found to be an optimal heating rate which the Bi2212 thin films exhibited excellent phase purity and crystalline properties, and their Tc.onset rose to about 95 K.In view of the effect of mismatch on growth mode, crystalline and superconducting properties of epitaxial Bi2212 thin films and based on the optimal preparation condition of Bi2212 thin films, a comparative experiment of Bi2212 films grown on LAO and MgO was designed and executed. The obtained results indicated that the interface orientations between the thin films and substrates were Bi2212[100](001)//STO[011](100), Bi2212[100](001)//LAO[011](100) and Bi2212[100](001)//MgO[010](100), respectively. The Bi2212 films deposited on STO and LAO substrates showed perfect epitaxial growth. Due to the emergence of a/b-axis oriented Bi2212 grain at higher sintering temperature, single extensionality of Bi2212 films grown on MgO substrates was destoryed. The phase formation temperature range of Bi2212 films grown on LAO and MgO substrate were 819℃ /65min-827℃/25min and 819℃/70min-824℃/38min, respectively and the optimal Tc.onset values were 96K and 92K respectively. Therefore, stress-strain caused by lattice mismatch between films and substrates directly effected the growth mode, crystalline state and superconducting properties of epitaxial Bi2212 thin films.The relation of superconducting properties and internal stress-strain state caused by thickness effect and mismatch between Bi2212 thin films and substrates were investigated by the variation patterns analysis of lattice constant of Bi2212 thin films. The results indicated that for Bi2212 thin films grown on MgO, STO and LAO, the tensile stress within in a-b plane and the lattice constant a/b were larger(i.e., c is smaller) than standard value (PDF79-2183, a=5.405A,b=5.401A, c=30.715A). With the increase in thickness of Bi2212 thin films grown on MgO substrates, the alb was reduced down from 5.508A to 5.450A(the c value was increased from 30.540A to 30.640A), meanwhile there were no obvious changes in the lattice constants of Bi2212 thin films grown on STO and LAO substrate with values a/b≈ 5.425A(c=30.603A) and a/b≈5.436A(c=30.346A), respectively. The Tc.onset of thin film showed a trend of initial increased and then decrease in value with the increase of average strain. The average strain was measured 7.433×10-4, whereas the Tc.onset value rose to 96K. |
PDF Full Text Request